A widely known form of burner nozzle, through which fuel may be discharged into a furnace chamber, includes a tip having a ring of bores or ports in the form of passages whose axes diverge from a common center. A particular form of such a tip is arranged to operate with a supply of steam to the ports, and a supply of oil into each port from another passageway that opens into the port. A mixture of steam and oil emerges from the ports, and it is essential that, within close tolerances, the ports be similar to each other in cross section and in the inclination of each to the axis of the nozzle. The present invention arose from a consideration of ways in which the similarity of the ports might be tested.